The present invention as well as the cited prior art deal with semiconductor switching devices including MCTs, MOSFETs, IGBTs and thyristors. To aid in explaining the different characteristics of these devices, a brief overview of the various types of devices and their properties is discussed.
Goals of semiconductor switching devices include high current capability per unit area of silicon, high switching speed, low on-resistance, and ease of control. Thyristors, such as the widely accepted silicon controlled rectifiers (SCRs), potentially possess the lowest on-resistance and also have high power handling capability. However, special techniques for control, slow switching speed, and attendant high switching losses limit their usefulness. Insulated gate bipolar transistors (IGBTs), which are easier to turn on and turn off, traditionally possess higher on-resistances than thyristors. Metal oxide silicon field effect transistors (MOSFETs), also called insulated gate field effect transistors (IGFETs), provide faster switching characteristics than SCRs, but typically have a higher on-resistance. This higher on-resistance diminishes their switching performance.
MOS-controlled thyristors (MCTs), a new class of power devices, combine thyristor current and voltage capability with MOS gated turn-on and turn-off. Currently, industry builds various types of MCTs. These are p-type or n-type, symmetric or asymmetric blocking, and various turn-on alternatives including direct turn-on by light. However, all MCTs turn off the thyristor by shorting out one or both of the thyristor's emitter to base junctions. Like their MOSFET ancestors, these devices characteristically possess a high on-resistance, thereby diminishing their switching performance.
In particular, the prior art MCT possesses a large physical separation between the active thyristor junction and the MOSFET channel region. This large physical separation creates "parasitic" resistance between these two structures. The parasitic resistance impairs the MOSFET's ability to shunt the turn-on current at the thyristor base to ground, thereby causing inefficient thyristor turn-off.